Pressure regulators such as those used in underwater breathing apparatus commonly employ the pressure differential between the ambient and a breathing chamber in the regulator to operate an air valve which supplies air to the breathing chamber. This is accomplished by mounting a flexible diaphragm across an opening in the wall of the breathing chamber and using the diaphragm to actuate the air valve. Since the mouthpiece is connected to the breathing chamber the diver breaths from the breathing chamber. In single hose regulators the diver also exhales through the breathing chamber to the ambient while in double hose regulators the exhaled gasses go directly to the ambient.
When the diver commences to inhale while the air inlet valve is closed, the pressure in the breathing chamber is reduced causing the diaphragm to be sucked into the breathing chamber and thereby open the air inlet valve. When the user exhales, the pressure in the breathing chamber increases to cause the diaphragm to move out and thereby to close the air inlet valve. In order to reduce the effort required to breath from such regulators it is common practice to design the regulator so that a portion of the inlet air travels as a jet directly into the mouthpiece tube, thereby to provide a so-called venturi effect which educts air from the breathing chamber and thus reduces the pressure in the breathing chamber. Consequently, the diaphragm is held in the pulled in position by the venturi action and holds the air inlet valve open. While such a venturi effect makes it easier for the user to inhale from the regulator, exhaling becomes more difficult inasmuch as the venturi action must be overcome before the air inlet valve can be closed. Accordingly, the amount of venturi action provided must be carefully adjusted for optimum inhalation and exhalation.
In the prior art regulators the amount of venturi action is at a maximum when the air inlet valve is fully open. However, it is while the air inlet valve is fully open that exhalation ordinarily occurs. On the other hand, the need for the venturi action is greatest when inhalation commences and the air inlet valve begins to open. Yet the air flow rate is low at this time wherefor the venturi action is also low.